Rocket body explosions also can be generated by nonchemical means, such as overpressure leading to propellant tank rupture. Overpressure may occur for a number of reasons, including propellant heating and failure of pressure relief valves. Explosions caused by nonchemical means are often less energetic than those caused by propellant mixing. Since explosions caused by overpressure cause no transient stresses, theoretically the propellant tank will tear along lines of weakness, generating few, if any, fragments, and the additional velocity imparted to any fragments should also be low (Fucke, 1993). However, the 1986 explosion of an Ariane third stage, which is believed to have been caused by overpressurization, produced a record number of cataloged fragments, and explosions generated by nonchemical means probably caused seven of the ten largest fragmentation events recorded (all with more than 225 cataloged fragments).
Launch vehicle builders have developed a number of methods to reduce a rocket body's potential for explosion. In general, the methods involve either (1) depletion burns after the rocket body separates from the spacecraft or (2) venting of residual propellant. Although these passivation measures will not eliminate propulsion-related breakup events (i.e., breakups that occur during rocket ignition and propulsion), such events are rare for orbital rocket bodies.
In depletion burns, the engine is reignited after completion of the staging process and operated under normal conditions until its propellant is depleted. In principle, depletion burns can shorten the rocket body's orbital lifetime, although past burns of some rocket bodies have increased orbital lifetime. (See the discussion of orbital lifetime reduction later in this chapter.) Such a maneuver typically requires using the rocket body's battery for power and its auxiliary thrusters for attitude control. To gain the maximum lifetime reduction from such a maneuver, the depletion burn should be carried out near the orbit's apogee; to prevent the contamination of nearby spacecraft, some rocket bodies may have to retain the capability to make such burns for several hours after staging. Currently, some rocket bodies are capable of performing depletion burns for a short time after spacecraft delivery, and most other rocket bodies would require only minor modifications to be able to perform depletion burns.
Venting of residual propellant can be achieved either by blowing the propellant out through valves or by evaporating and venting it. To vent residual propellant, a rocket body generally requires pressure relief valves (usually activated by firing pyrotechnic devices) and venting pipes. The advantage of venting is that it does not require reignition or auxiliary thrusters. The Ariane rocket bodies (see Box 7–) now vent their residual propellant.
Residual propellant from the main rocket engines is not the only